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Somersetite, Pb8O(OH)4(CO3)5, a new complex hydrocerussite-related mineral from the Mendip Hills, England

Published online by Cambridge University Press:  15 May 2018

Oleg I. Siidra*
Affiliation:
Department of Crystallography, St Petersburg State University, University Embankment 7/9, 199034 St Petersburg, Russia Nanomaterials Research Center, Kola Science Center, Russian Academy of Sciences, Apatity, Murmansk Region, 184200, Russia
Diana O. Nekrasova
Affiliation:
Department of Crystallography, St Petersburg State University, University Embankment 7/9, 199034 St Petersburg, Russia
Rick Turner
Affiliation:
The Drey, Allington Track, Allington, Salisbury SP4 0DD, Wiltshire, UK
Anatoly N. Zaitsev
Affiliation:
Department of Mineralogy, St. Petersburg State University, University Embankment 7/9, 199034 St Petersburg, Russia Core Research Laboratories, Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London SW7 5BD, UK
Nikita V. Chukanov
Affiliation:
Institute of Problems of Chemical Physics, Chernogolovka, Moscow region, 142432, Russia
Yury S. Polekhovsky
Affiliation:
Department of Mineral Deposits, Faculty of Geology, St. Petersburg State University, University Embankment 7/9, St. Petersburg 199034, Russia
John Spratt
Affiliation:
Core Research Laboratories, Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London SW7 5BD, UK
Mike S. Rumsey
Affiliation:
Mineral and Planetary Sciences Division, Earth Sciences Department, Natural History Museum, Cromwell Road, London SW7 5BD, UK
*

Abstract

The new mineral somersetite, has been found at Torr Works (‘Merehead quarry’) in Somerset, England, United Kingdom. Somersetite is green or white (typically it is similar visually to hydrocerussite-like minerals but with a mint-green tint), forms plates and subhedral grains up to 5 mm across and up to 2 mm thick. In bi-coloured crystals it forms very thin intergrowths with plumbonacrite. The empirical formula of somersetite is Pb8.00C5.00H4.00O20. The simplified formula is Pb8O(OH)4(CO3)5, which requires: PbO = 87.46, CO2 = 10.78, H2O = 1.76, total 100.00 wt.%.

The infrared spectrum of somersetite is similar to that of plumbonacrite and, to a lesser degree, hydrocerussite. Somersetite is hexagonal, P63/mmc, a = 5.2427(7), c = 40.624(6) Å, V = 967.0(3) Å3 and Z = 2. The eight strongest reflections of the powder X-ray diffraction (XRD) pattern [d,Å(I)(hkl)] are: 4.308(33)(103), 4.148(25)(104), 3.581(40)(107), 3.390(100)(108), 3.206(55)(109), 2.625(78)(110), 2.544(98)(0.0.16) and 2.119(27)(1.0.17). The crystal structure was solved from single-crystal XRD data giving R1 = 0.031. The structure of somersetite is unique and consists of the alternation of the electroneutral plumbonacrite-type [Pb5O(OH)2(CO3)3]0 and hydrocerussite-type [Pb3(OH)2(CO3)2]0 blocks separated by stereochemically active lone electron pairs on Pb2+. There are two blocks of each type per unit cell in the structure, which corresponds to the formula [Pb5O(OH)2(CO3)3][Pb3(OH)2(CO3)2] or Pb8O(OH)4(CO3)5 in a simplified representation. The 2D blocks are held together by weak Pb–O bonds and weak interactions between lone pairs.

Type
Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Juraj Majzlan

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